Theiler's murine encephalomyelitis viruses (TMEV) strains can be divided into two subgroups on the basis of biological activities. The GDVII subgroup strains cause an acute, lethal neuronal disease with no virus persistence, while DA (TO) subgroup strains are less neurovirulent and produce a chronic demyelinating infection. Our major goal is to define viral genes and gene products critical for determining the virus' biological activities (neurovirulence, neurotropism, virus persistence). The TMEV system is a valuable one for such studies because of: the use of a mouse as an experimental host, the large amount of molecular information that will soon be available about this small virus with its small genome, the potential use of recombinant infectious cDNA from strains of both subgroups. We have a panel of TMEV neutralizing monoclonal antibodies (mAbs) and we are generating TMEV mutants resistant to them. We will sequence the epitopes of the neutralizing mAbs by RNA primer extension. The neutralization sites will be localized on the genome and virion structure. The importance of epitopes to TMEV's biological activities will be determined by observing the effect of inoculating mutants into mice. We have almost the whole of DA strain cloned and sequenced. We now plan to: clone and sequence FA strain (a member of the GDVII subgroup) and prepare DA and FA infectious cDNA. Recombinant infectious cDNA studies, guided by comparisons between sequences from both subgroups and by our mutant studies, will be performed to delineate parts of the genome critical for TMEV's biological activities. These studies may elucidate mechanisms of neuron tropism and injury relevant to motor neuron disease.